mjhjmtu

I use a def as an alias like so:

defaliasfoo

later on I would like to check if alias is an alias for foo. (X is an alias of Y means the current definition of X is defXY).

See the MWE below for what I tried:

MWE:

documentclassarticle
%usepackagetrace
deffoobar
defbarbar

newcommandisFoo[1]
% expandafterifexpandafternoexpandaliasnoexpandfoo %false positives
% expandafterifxaliasfoo %this one compares text
% expandafterifxexpandafternoexpandaliasnoexpandfoo %false negatives
 It is a textbackslash foo #1
 else
 Not a textbackslash foo #1
 fi

begindocument
 defaliasfoo
 isFooab

 defaliasbar
 isFooxy

 defbarbaz
 isFooxy
enddocument

The idea behind the expandafter/noexpand code is to expand alias exactly once and then compare the two tokens without any further expansion to avoid comparing their return values.

You could also just set up another alias for foo and compare alias to that with ifx.

documentclassarticle

% deffoobar
% defbarbar

defknownaliasforfoofoo
newcommandisFoo[1]%
 ifxaliasknownaliasforfoo
 It is a textbackslash foo #1%
 else
 Not a textbackslash foo #1%
 fi


begindocument
 defaliasfoo
 isFooab

 defaliasbar
 isFooxy

 defbarbaz
 isFooxy
enddocument

You can uncomment either or both of the commented lines and the output would remain the same.

With expl3:

documentclassarticle
usepackagexparse

ExplSyntaxOn
NewExpandableDocumentCommandifisaliasTFmmmm
 % #1 = token to test
 % #2 = macro
 % #3 = true case
 % #4 = false case
 ted_ifisalias:nnTF #1 #2 #3 #4 
 

prg_generate_conditional_variant:Nnn tl_if_empty:n f T,F,TF,p 
prg_generate_conditional_variant:Nnn tl_if_single:n o T,F,TF,p 

prg_new_conditional:Nnn ted_ifisalias:nn T,F,TF,p 
 
 token_if_macro:NTF #1
 % the first argument is a macro
 tl_if_empty:fTF token_get_arg_spec:N #1 
 % the macro has no arguments
 tl_if_single:oTF #1 
 % the expansion is a single token
 exp_last_unbraced:No token_if_eq_meaning:NNTF #1 #2
 % the first level expansion of #1 is the same as #2
 prg_return_true:
 
 % false
 prg_return_false:
 
 
 % false
 prg_return_false:
 
 
 % false
 prg_return_false:
 
 
 % false
 prg_return_false:
 
 
ExplSyntaxOff

newcommandfooWhatever
newcommandaliasfoo
newcommandnotaliasA[1]foo
newcommandnotaliasBfoofoo

begindocument

ifisaliasTFaliasfooAliasNot alias

ifisaliasTFfoofooAliasNot alias

ifisaliasTFnotaliasAfooAliasNot alias

ifisaliasTFnotaliasBfooAliasNot alias

enddocument

The following should do what you want.

documentclassarticle

usepackagepdftexcmds

defaliasfoo
makeatletter
newcommandcheckalias[2]
 %
 expandafterexpandafterexpandafter
 checkalias@aexpandafterexpandafterexpandafter
 expandafterdetokenizeexpandafter#1#2%
 
newcommandcheckalias@a[2]
 %
 expandafterifnum
 expandafterpdf@strcmpexpandafterdetokenize#2#1=0
 expandafter@firstoftwo
 else
 expandafter@secondoftwo
 fi
 
makeatother

begindocument
checkaliasaliasfoo
 still an aliasno longer an alias
defaliasbar
checkaliasaliasfoo
 still an aliasno longer an alias
enddocument

It doesn't check whether the first argument would grab any arguments though and is therefore not as safe as @egreg's check. I don't think it is worth it implementing the necessary checks without expl3 though.